1. Field of the Invention
The present invention relates to a vehicle speed measuring apparatus for measuring speed of a vehicle.
2. Discussion of Background Art
Vehicle speed is utilized for various controls of a vehicle. Generally, vehicle speed is measured by calculating detection values from wheel speed sensors. For example, in an antilock braking system which prevents wheel lock-up during braking or a vehicle behavior control apparatus, vehicle speed is measured by calculating detection values detected by wheel speed sensors. Japanese Laid-open Patent Application No. 2001-47998 (claims 2 and 3, columns 0031 and 0032, FIG. 5, etc.) discloses that vehicle speed is obtained by selecting the fastest wheel speed of four wheel speeds detected from the respective wheel sensors or by calculating the average wheel speed of the driven wheels.
However, a wheel speed sensor calculates wheel speed by counting the number of rotations of the tire (wheel) in a certain period of time. For this reason, in a case where the vehicle is equipped with deformed tires, or in a case where the tire inflation pressure has been changed, the tire diameter per se varies. This leads to a change in the wheel speed, and thus results in a change in the vehicle speed calculated from the wheel speed. The same can be said for a vehicle sensor which measures vehicle speed by detecting the rotational speed of the output shaft of the transmission, and a change in the tire diameter results in a change in the vehicle speed.
With the foregoing drawback of the art in view, the present invention mainly seeks to provide a vehicle speed measuring apparatus which can measure vehicle speed without being affected by a change in the tire diameter.